These studies seek to assess the effects of environmental agents on spermatogenesis, male accessory glands and male reproductive capacity. Mechanisms of toxicity are studied and new approaches to toxicity testing are proposed and validated in order to gain more reliable extrapolation of laboratory data to man and improve estimates of human risk. The following studied are ongoing: (1) defining the role of germ cell DNA repair as an indicator and modifier of toxicity; (2) modeling the various factors which affect testicular toxicity; and (3) investigating the toxication-detoxication of polycyclic aromatic hydrocarbons in gonadal and prostate tissues using both in vitro perfusion and cell free preparations. To date, selected mutagens, trace metals, the nematocide, dibromochloropropane diesel, trimethylamidoborane, and tetrachlorodibenzo-p-dioxin have been studied. Alkaline elution analysis of germ cell DNA, unscheduled DNA synthesis, and determination of germ cell nicotinamide adenine dinucleotide level appear to be reliable indicators of DNA damage and unscheduled DNA repair activity, as well as predictors of toxicity. The interrelationship of the blood-testis barrier, toxication-detoxication reactions, and DNA repair have been described.